The present invention relates to a hydraulic tappet designed for assembly on a drive for controlling a valve on an internal combustion engine.
Hydraulic tappets designed for assembly on a drive for controlling a valve on an internal combustion engine are known usually to comprise a first element, moving in relation to the crankcase, and a second element moving axially in relation to the first. Between the two elements there is formed a first chamber of varying volume having an inlet duct for drive fluid, the aperture of the inlet duct being controlled by an on-off member in such a manner that the fluid flowing into the chamber causes one of the elements to move axially in relation to the other, so as to vary the volume inside the chamber and so recover any slack on the gear chain on the drive.
The first variable-volume chamber usually communicates through appropriate ducts with a suitable drive fluid source.
The drawback of tappets of the aforementioned type is that they operate poorly at the initial engine operating stage, since the oil contained in the chamber, when the engine is off, seeps out or leaks out of the chamber between the mating surfaces of the first and second moving elements, and also due to the fact that, at the initial operating stage, the oil pressure on the engine is so low as to be incapable of ensuring an adequate oil supply to the chamber itself.
In some known tappets, the aforementioned drawbacks have been partially overcome by providing a second drive fluid chamber communicating hydraulically with the first and in which second chamber a given amount of drive fluid is preserved, retained or maintained even when the engine is left idle, thus enabling the first chamber to be filled more easily, when the engine is started up, than in the foregoing arrangement.
Even on these tappets, however, long-term operation of the engine may result in oil leaking from the second chamber to the extent that operation of the tappet is seriously jeopardized. Furthermore, leakage between the mutually-sliding mating surfaces on the two sliding elements is not prevented by providing appropriate sealing members between the surfaces.